Ferritic stainless steel (steel sheet), which is economical and excellent in terms of corrosion resistance, is used in various applications such as building materials, transportation instruments, home electrical appliances, kitchen equipment, and automobile parts, and the range of its application has been expanding in recent years. Of these applications, in applications in which surface appearance is important such as interior construction materials, bodies and doors of home electrical appliances, kitchen equipment, and molding for automobiles, good surface appearance is particularly emphasized.
Good surface appearance requires high surface glossiness and the absence of roping. Surface brightness varies depending on the color tone of a surface and the degree of light reflection, which vary depending on the fine irregularities of the surface; the smoother the sheet surface, the higher the brightness. In order to increase the brightness, it is necessary to reduce the fine irregularities of a steel sheet surface typified by rolling-induced defects (oil pits and scratched marks generated by the transfer of the polishing marks of rolls) in a cold rolling process. Roping is a defect unique to ferritic stainless steel and generated as irregularities extending in the rolling direction.
Moreover, when a forming process such as pressing is performed before use, no generation of ridging and surface roughening is also necessary. Ridging is a defect unique to ferritic stainless steel and generated as irregularities extending in the rolling direction. Surface roughening is caused by undulation of coarse crystal grains. Ridging or surface roughening generated in a forming work process needs to be removed by polishing, which results in a considerable increase in the manufacturing load and manufacturing costs.
In order to satisfy such requirements, regarding a technique of obtaining a cold-rolled stainless steel sheet excellent in terms of surface quality before and after a forming process, Patent Literature 1 discloses a ferritic stainless steel sheet having less planar anisotropy, being excellent in terms of ridging resistance and surface roughening resistance, and being characterized by subjecting steel containing, by mass %, C: 0.005% to 0.100%, Si: 0.01% to 2.00%, Mn: 0.01% to 2.00%, P: less than 0.040%, S: 0.03% or less, Cr: 10% to 22%, Al: 0.0005% to 0.2000%, and N: 0.005% to 0.080% to, as a heat treatment process after a hot rolling process, preliminary annealing and subsequently to main annealing, or to soaking treatment and further to partial transformation heat treatment at a high temperature of 900° C. to 1100° C. or more, or to cold rolling before heat treatment. Patent Literature 1 does not refer to surface gloss; however, since recrystallization of the ferrite phase is progressed by sufficient soaking time, softening occurs and the steel sheet surface tends to be deformed. Thus, the above-described rolling-induced defects are generated, which results in a deterioration in surface gloss. In addition, in Patent Literature 1, since recrystallization is sufficiently progressed, surface irregularities cannot be prevented from generating in a cold rolling process with enough tension, which results in generation of roping.
Patent Literature 2 discloses a ferritic stainless steel sheet that is excellent in terms of ridging resistance, workability, and surface brightness and that is obtained by controlling the sheet-thickness-direction length of colonies to be 30% or less of the thickness of the sheet. However, the method of controlling ferrite colonies in Patent Literature 2 does not reduce roping and the phenomenon of distortion of a reflected image on the surface visually observed still occurs.
Patent Literature 3 discloses a technique in which brightness is improved by decreasing the amount of oil drawn in order to reduce occurrence of oil pits and by minimizing the transfer of concave-convex patterns on the surfaces of rolls as a result of using hard low-surface-roughness work rolls in a cold rolling process. However, while the technique of Patent Literature 3 can remove rolling-induced surface defects, it cannot solve a problem of surface defects due to a raw material such as roping, ridging, and surface roughening.